During regression of hormone-dependent mammary tumors produced with a variety of procedures (ovariectomy, DBCAMP, choleratoxin or antiestrogens) the cAMP receptor level was enhanced and the estrogen receptor level was depressed in an antagonistic manner. Within 6 hours post ovariectomy or DBcAMP treatment, the in vitro synthesis of several polypeptides instructed by poly(A)+RNAs of the tumors was either depressed or stimulated and the same new set of genes are expressed in both ovariectomy- and DBcAMP-induced regressing tumors. The same changes in the translation products were also demonstrable in the tumor slices preincubated with cAMP in vitro, mimicking the regressing tumors in vivo and this cAMP effect was abolished when 17 Beta-estradiol was simultaneously presently with cAMP. Thus, mammary tumor growth is subject to transcriptional control and that the antagonism between cAMP and estrogen is involved in this genetic event.